When high tensile steel is welded, preheating/interpass temperature should be controlled strictly with a view point of preventing cold cracking of a weld metal portion, which lowers operation efficiency. In recent years, the strength of steels used for welded structures has become higher and a demand for higher strength has been increased also in weld metals (for example, HT780: high tensile 780 MPa grade).
Such increase in the strength tends to lower the cold cracking resistance and it is necessary to improve the cold cracking resistance. Particularly, since gas shield arc welding using a flux-cored wire has excellent welding workability, a technique for ensuring cold cracking resistance has been demanded in the weld metal formed by the welding method.
It is supposed that the cold cracking described above is attributable to segregation of diffusive hydrogen to grain boundaries to lower the strength at the grain boundaries (this is hereinafter referred to as “hydrogen embrittlement”), and for improvement of cold cracking resistance it is important how to decrease diffusion hydrogen.
In view of the above, for improving the cold cracking resistance of the weld metal, it is necessary to lower the susceptibility of the weld metal to hydrogen embrittlement and various techniques have been proposed from such a view point.
For example, Patent literature 1 discloses a technique of dispersing a Mo carbide (Mo-containing carbide) of high hydrogen trapping performance in a weld metal thereby preventing cold cracking. However, since it is necessary for this technique to adopt a specific welding method of applying submerge arc welding from the inside after abutting steel materials, it is not applicable generally to welding of steels.
Further, Patent literature 2 discloses a technique of preventing cold cracking of welded joints by dispersing a Si—Mn—Ti—Al composite oxide which is effective to trap diffusive hydrogen into a weld metal thereby preventing cold cracking in the welded joints. However, the level of the strength intended in this technique is 588.4 MPa or more in terms of tensile strength, and it cannot be said that a sufficient strength can be ensured.
Patent literature 3 proposes a technique of improving the cold cracking resistance by decreasing the amount of diffusive hydrogen, as well as properly controlling the strength and the chemical component composition. However, since a satisfactory strength level undergoes the effect of components, application cite is limited also in this technique upon actual operation.
Meanwhile, there are also proposed a technique of improving the cold cracking resistance by occluding diffusive hydrogen in a weld metal by addition of V and forming fine carbides thereby fixing carbon in the weld metal (for example, Patent literatures 4 and 5), a technique of imparting low temperature toughness, proof stress, and cracking resistance together by specifically controlling the flux components (for example, Patent literatures 6 and 7), etc. While each of the techniques described above intends to improve the cold cracking resistance, since the amount of hydrogen in the weld metal may possibly be increased by various factors in actual welding operation, it is necessary to improve the resistance to hydrogen embrittlement susceptibility in a more essential approach.
Further, Patent literature 8 also proposes a technique of making the strength and the toughness compatible by finely controlling the form of oxides containing Ti, Si, etc. and developing fine acicular ferrite structures on the oxides as nucleation sites. However, this technique has no consideration on the cold cracking resistance.